The present invention is directed to optical discs comprising a substrate, a metallized layer and a coating, and more particularly to such optical discs wherein the coating is formed from a radiation-curable composition and has an advantageous combination of physical properties.
Optical disk technology is well known in the art. Generally, in the formation of an optical storage disk such as a compact disk (CD), a molten transparent plastic is injection molded to form a substrate. On one surface of the substrate, a replica of pits and lands contained in the master mold is formed. A highly reflective layer of metal, for example, gold, copper, silver or aluminum, is applied to the substrate surface which contains the pits and lands to allow the surface to reflect a laser beam of an optical disk reader such as a CD player. The reflective layer is commonly made of aluminum and generally is applied at a thickness of about 50 to 100 nanometers by vacuum deposition or the like. Conventionally, the metallized substrate is then provided with an ultraviolet (UV) curable coating by spin coating, electro-spray techniques, vacuum deposition, or the like. Typically, when the coating is applied by spin coating, a bead of a UV curable coating is applied to the center inner diameter of the metallized substrate as the substrate is rotated to coat the entire surface. The coating is then cured and may be subsequently printed if desired. Generally, the UV cured coating protects the metallized substrate from abrasion and humidity.
The Wolf et al U.S. Pat. No. 4,652,498 discloses an optical medium comprising a transparent substrate having an information-bearing surface, a specularly reflective layer and a protective layer formed from a polymeric network of ethylenically unsaturated ingredients such as polyacryloyl and polymethyacryloyl materials. A compound that provides phosphoric acid functionality is also included. The Namba et al U.S. Pat. No. 5,161,150 discloses optical recording discs having a protective layer formed of a radiation-curable compound and a photo-polymerization sensitizer. Suitable radiation-curable compounds are disclosed as including oligo ester acrylates used in combination with radiation-curable monomers, examples of which include various acrylates, diacrylates and triacrylates. The Dijkstra et al U.S. Pat. No. 4,188,433 discloses optical disks including a cover layer formed from an ultraviolet-curing lacquer containing a protic mixture of acrylic acid esters. Epoxy acrylates and urethane acrylates are also disclosed as suitable.
Generally, conventional UV-curable coating compositions have provided optical disks such as CD""s with varying degrees of protection against abrasion and humidity. It is also important that the cured coating composition remains colorless and transparent, is easily printable and may be produced at relatively low cost. Those skilled in the art will recognize that not only are the UV coating composition""s final physical properties important in providing an acceptable optical disk product, various properties of the uncured composition are also important. For example, the uncured composition must exhibit a low viscosity and good flow to provide good, uniform coatings on the metallized substrate prior to cure. The cure speed of the coating composition is also important in providing a product which may be readily mass produced. In many conventional coating compositions, these desired features compete with one another so that improving one property, for example the viscosity of the uncured composition, disadvantageously effects another property, for example the hardness of the cured composition. Accordingly, there is a continuing need to develop coating compositions for optical disks which optimize these and additional, sometimes competing, requirements.
Accordingly, it is an object of the present invention to provide optical disks which overcome disadvantages of the prior art. It is a more specific object of the present invention to provide optical disks which include a coating formed from a radiation-curable composition which provides an advantageous combination of properties. It is a further object of the invention to provide optical storage disks such as compact disks which exhibit an advantageous combination of physical properties. It is yet an additional object to provide optical disks which may be produced at an improved rate as compared with conventional optical disk products.
These and additional objects are provided by the optical disks according to the present invention. More particularly, the disks comprise a substrate, a metallized layer and a coating which is formed from a radiation-curable composition. In one embodiment, the radiation-curable composition comprises a first component selected from the group consisting of alkanediol diacrylates, alkanediol dimethacrylates, alkene glycol diacrylates, alkene glycol dimethacrylates, and mixtures thereof, and a second component comprising an acrylate- or methacrylate-containing alkoxylated bisphenol A compound. The radiation-curable compositions further include a photoinitiator which initiates curing of the first and second components to form a hard protective coating, and an adhesion promoter for promoting adhesion of the coating to the disk, for example to the metallized layer. The adhesion promoter comprises acrylated sulfonic acid, methacrylated sulfonic acid, acrylated sulfonic acid anhydride, methacrylated sulfonic acid anhydride, acrylated carboxylic acid, methacrylated carboxylic acid, acrylated carboxylic acid anhydride, methacrylated carboxylic acid anhydride, acrylated phosphoric acid, methacrylated phosphoric acid, or mixtures thereof.
The optical disks according to the present invention containing a coating formed from the described radiation-curable compositions exhibits a good combination of physical properties, including good hardness, good water resistance and good colorless transparency. Additionally, the radiation-curable compositions used to form the coating in the optical disks of the present invention are easily applied to fully and uniformly coat the metallized substrate and are quickly cured to facilitate mass production of the optical disks.
These and additional objects and advantages will be more fully apparent in view of the following detailed description.